Located in the eastern half of the old north wing, the Old Senate Chamber is a semicircular room 75 feet in diameter covered by a half dome. It was designed by Benjamin Henry Latrobe and constructed by him and his successor, Charles Bulfinch, in 1815-1819. Latrobe modeled the chamber after similar rooms he had seen in Paris, which he considered ideally suited for hearing, speaking and seeing. He employed marble columns based on Grecian examples to support the visitor's gallery along the eastern wall. In 1828 Charles Bulfinch added a second gallery supported by slender cast-iron columns along the curving western wall.
The Senate met in this room from 1819 until its new chamber was ready in 1859. The next year the Supreme Court took over and remained here until moving across the street in 1935. For the next 40 years the room was used as a meeting or entertainment room. It was restored in 1976 to commemorate the history of the Senate. - US Architect of the Capital